Sandara Hill is a volcanic plug standing some 200 m above the desert floor. The Nubian Sandstone at its foot dips steeply outwards for 50 m from the contact before becoming horizontal again. The principal rock type is a nosean-bearing phonolite. A further plug of similar composition is located 5 km northeast of Sandara Hill. Scattered outcrops of trachytic and phonolitic rocks cover several km2 around both hills. Another hill of phonolite lies some 15 km northwest of Sandara Hill and two further hills of trachyte and phonolite 8-10 km to the south-southeast. The trachyte of the first occurrence contains a little quartz while the second hill consists of two concentric rings of phonolite which contains some sodalite.